I. Field of the Invention
The present invention relates generally to valve actuators and, more particularly, to an electronically controlled valve actuator.
II. Description of the Prior Art
There are a number of previously known valve actuators having an actuating member, typically a shaft, which is movable or rotatable from a first position in which the valve is closed and a second position in which the valve is open. A number of these previously known valve actuators also include a reversible electric motor mechanically connected to the valve actuating member for moving the actuating member, and thus the valve, between its closed and open position and vice versa. These previously known motor driven valve actuators, however, suffer from a number of disadvantages.
When a valve is in a closed position, a relatively great amount of torque must be initially supplied by the motor of these previously known valve actuators in order to break the valve open in contrast to the average dynamic torque required for moving the valve actuating member from its closed and to its open position. The previously known valve actuators have employed a mechanical hammer blow which impacts the actuator for a period in the order of ten miliseconds. The effectiveness of such an impact, however, is dimensioned by the amount of valve load inertia thus requiring even greater impacts from the mechanical hammer to ensure that the valve is broken open from its closed position.
In order to accomplish the necessary hammer blow to break the valve open from its closed position, it has been necessary for the previously known valve actuators to employ specially designed torque motors which are capable of creating the necessary hammer blow. Such torque motors, however, are very expensive in both acquisition and maintenance costs.
A still further disadvantage of these previously known motor driven valve actuators is the difficulty encountered in the initial set up of the valve. Ideally, the simple activation of a switch button to move the valve from its open position and to into its closed position, or vice versa, is most desirable. However, in order to accomplish this it has been necessary to undergo tedious and time-consuming mechanical adjustments and even gear changes on the valve actuator to ensure that the motor moves the valve completely between the open and closed position and then deactivates. In addition, many of these previously known motor driven valve actuators have required adjustments of potentiometers or other variable electrical components in order to initially set up the valve. Such adjustments, however, are also tedious and time-consuming and, moreover, are subject to change over age thus requiring readjustment.
A still further disadvantage of these previously known motor driven valve actuators is that with such actuators the valve can be actuated only from its closed position and to its open position, or vice versa, and that it was not possible to stop the valve at selected or predetermined intermediate positions. In many types of valve applications, however, it would be desirable to open the valve to one or more predetermined intermediate positions between its fully open and fully closed position.